Over the past few years, improvements to conventional lead acid batteries/energy storage devices, have evolved. One such improvement involves the substitution for a traditional lead negative electrode by an activated carbon electrode such as that, for example, described in Applicant's U.S. Pat. No. 7,998,616. The electrode is an assembly is composed of a copper current collector, laminated on both surfaces with a paraffin impregnated expanded graphite shield and an outer activated carbon sheet. The upper edge of the electrode current collector features a projecting tab for current collection. This construction is referred herein to as a “standard packet”.
Manufacture of low resistance standard packets with a leak-free expanded graphite foil negative electrode structure has proven challenging. Initial efforts to reduce leaks focused on the elimination of pin-holes that developed in the paraffin impregnated expanded graphite foil. The pin-holes were attributed to contamination from, ash particles, in the graphite raw material which reacted with the sulfuric acid electrolyte. The voids/small pin-holes were created from the dissolving of the ash particles in the graphite foil shield material in the presence of sulfuric acid. Batteries made from expanded graphite foil containing ash particles had an average AC impedance of 9.37±1.75 mΩ. The successful elimination of the contamination (ash particles) from the expanded graphite foil raw material solved the perceived pin-hole problem but lead to significantly higher contact resistance. Batteries made with ash free expanded graphite foil had an average AC impedance of 19.21±6.71 mΩ, an impedance increase of 2×.
In a standard packet laminated lead/activated carbon negative electrode, five components contribute to resistance due to its layered structure (from top to middle). The contributors to the cumulative resistance are: a) bulk resistance of the activated carbon sheet; b) interface resistance between the carbon and the impregnated graphite sheet; c) bulk resistance of the impregnated expanded graphite foil; d) interface resistance between the impregnated expanded graphite foil and the copper current collector; and e) bulk resistance of the copper current collector. The bulk resistance of the activated carbon sheet, impregnated graphite sheet and copper current collector are controlled by the materials electrical properties. The interface/contact resistance between the copper current collector and the impregnated expanded graphite foil shield elements has been identified as the potential source of variation in the laminate stack up.
Efforts to lower the standard packet resistance led to measuring the contributions of the bulk resistances of the individual components and assemblies. Measurements were taken the thickness plane using a current of 10 amps and measuring the voltage drop across the electrodes. The bulk resistance of the copper current collector is extremely small when compared to the other components in the laminate and, therefore, its contribution was disregarded. The resistance of the impregnated expanded graphite foil shield was determined to be 0.285+/−0.09 mΩ and the resistance of the activated carbon sheet was found to be 4.47+/−0.16 mΩ. Overall, the standard packet structure consisting of stacked activated carbon, impregnated expanded graphite foil and copper sheet without an edge laminating adhesive, a construct having no air trapped between the copper sheet and the impregnated graphite shield, should have exhibited resistance of 9.59+/−0.17 mΩ. But the resistance of a standard packet was found to be significantly higher—43.07+/−16.07 mΩ.
While it was believed that that by successfully eliminating the pin-hole formation, would prove beneficial, surprisingly, it was observed that air entrapment inside the standard packet during lamination significantly increased. The increased entrapped air within the then-sealed laminated standard packet resulted in significantly higher contact resistance, and particularly that between the copper current collector and the paraffin impregnated expanded graphite foil layers.
Efforts were then dedicated to minimize the quantity of entrapped air accumulated during lamination but met without success. The entrapment of air was minimized and the resistance of the laminate was reduced to 10.44+/−0.68 mΩ. However, the entrapped air could not be eliminated. Furthermore, in addition to the undesirable increased packet resistance, the variable amount of entrapped air also caused manufacturing difficulties in respect to the force required to compress the cell to the size of the case opening. Because entrapped air acts like a spring during compression, if the entrapped air volume is too large it causes glue line failure during compression leading to the escape of air from and electrolyte penetration into the packet. The resulting batteries made from these packets exhibited a wide resistance variation and concomitantly non-uniform performance.
Therefore, a need exists for a solution for elimination of entrapped air in the activated carbon negative electrode and mitigation of the increased resistance problem.